Recently, it has attracted attention to reform a liquid with nanobubbles. On the other hand, it is strongly desired to improve the mileage or combustion efficiency of a diesel engine and a gasoline engine and it is also strongly desired to reduce the amount of harmful substance in an exhaust gas to be discharged. Therefore, it is desired to reform the fuel and to make the fuel capable of high performance.
For example, in the diesel engine, NOx is generated as air and oxygen coexist in a high temperature condition when combustion occurs in the engine cylinder. As the way to reduce the generation of the NOx, it can be cited to complete the combustion in a short period of time, but it causes an increase of generation of PM (Particulate Matter). In order to avoid these issues, as the way to prevent the temperature increase inside the cylinder, which causes the generation of the NOx, while avoiding the combustion in the short period of time, it has been considered to utilize a water-in-oil (W/O) type water emulsion fuel.
By adopting the water emulsion fuel, it is expected (a) to prevent the temperature increase in the cylinder due to the latent heat of vaporization of water when combustion occurs, and further (b) to assist in the fuel atomization by the micro explosion of water.
As the prior art to manufacture the emulsion fuel, a method of utilizing a surfactant is disclosed (Patent Document 1). Further, another technology is disclosed, in which high-frequency vibration generated by a vibrator made of a magnetostrictive material and a piezoelectric device, or a giant magnetostrictive material is applied, then cavitation is caused in a mixture liquid of the fuel and water, and a high-density emulsion fuel containing ultra fine water particles dispersed and mixed in the fuel is produced (Patent Document 2). And a fuel modification device for the internal combustion engine is disclosed, the device comprising: a modification tank storing gasoline and water inside; and fine bubble introduction means for emulsifying the mixture liquid of the gasoline and water (Patent Document 3), but it is inferred that fine water particles are stabilized in the fuel for the internal combustion engine by the effect of the electrostatic polarization in a similar manner of the surfactant (Patent Document 3) probably because the fine bubbles are generated by utilizing the ultrasonic wave (Patent Documents 4 and 5).    [Patent Document 1] Japanese Patent Application Publication No. 2004-67913    [Patent Document 2] Japanese Patent Application Publication No. 2004-76608    [Patent Document 3] Japanese Patent Application Publication No. 2006-241243    [Patent Document 4] Japanese Patent Application Publication No. 2003-334548    [Patent Document 5] Japanese Patent Application Publication No. 2004-121962